[#391] string_length: Remove use of strnlen and use consistent length type.
[#391] string_length: Remove use of strnlen and use consistent length type.

* Removes reference to strnlen as per #391

* Use ptrdiff for string_length interfaces for #391

bashrc-real/GSL,neilmacintosh/GSL,kernhanda/GSL,garyfurnish/GSL,martinmoene/GSL,Josaphat/GSL